There is an omnipresent need in the oil industry for rapid, high-volume liquid/liquid separation in which one of the liquid phases is conventional crude oil or "syncrude" produced from tar sands or oil shale and the other is water or brine. Oil and water are, of course, immiscible, however, an aqueous phase is frequently present in produced oil as a highly dispersed, discontinuous phase. The mixture is referred to as an emulsion. The source of this aqueous phse is formation water and/or condensed steam used in secondary and tertiary recovery.
The removal of entrained or emulsified water from oil is a long-standing problem, as evidenced by the following patents and publications: U.S. Pat. Nos. Prestridge, 3,772,180, 1973; 3,847,775, 1974: 4,116,790, 1978; 4,308,127, 1981; Waterman, L. C., Chem. Eng. Progress, 61 (10), 51-57 (1965); Sjoblom, G. L. and Goren, S. L., I&EC Fund., 5 (4), 519-525 (1966). One of the more widely used methods of performing this separation utilizes high voltage electric fields. Two mechanisms apparently operate to bring about coalescence of droplets of relatively polar water or brine in a non-polar medium, such as oil, with the force of an electric field. Firstly, the water droplets may acquire a net charge by direct contact with a charging electrode or through convective transfer of charge from the electrode by the oil. An attractive force will exist between water droplets which have acquired opposite charges. These attractive forces promote coalescence of the droplets. Secondly, water droplets in an electric field will become polarized by alignment of the polar water molecules with the external field and through the redistribution of mobile cations and anions within the water droplet. Attractive electrostatic forces between oppositely charged regions of neighboring water droplets promote their coalescence. The relative importance of these two mechanisms is evidently determined by the physical and chemical properties of the two phases. Of particular importance is the electrical conductivity of the oil. The mechanism of electrically enhanced coalescence has been discussed extensively in scientific literature. Refer to Pearce, C. A. R., Brit. J. of Appl. Phys., 5, 136-143 (1954); Allan, R. S. and Mason, S. G., Trans. Far. Soc., 57, 2027-2040 (1961); Pohl, H. A., J. Appl. Phys., 22 (7), 869-871 (1951); Bailes, P. J. and Larkai, S. K. L., Trans. I. Chem. E., 59, 229-237 (1981); Sadek, S. E. and Hendricks, C. D., I&EC Fund., 13, 139-142 (1974).
The background art in electrically enhanced coalescence is replete with the many variations for effective application of electric fields to the resolution of mixtures of polar liquids dispersed in non-polar liquids. One variation which is especially pertinent to the present invention is that of the degrading electric field. This concept forms the basis of U.S. Pat. No. 4,126,537 (1978), F. L. Prestridge and U.S. Pat. 4,308,127 (1981), Prestridge and Longwell. In the embodiment of electrically enhanced coalescence described in these disclosures, the voltage gradient between fixed electrodes is decreased in the direction of fluid flow by gradually widening the distance between the electrodes. The configuration of the electrode system is such that the emulsion to be treated is first exposed to a high electric field strength created by closely spaced electrodes, then exposed to progressively lower electric field strengths as the distance between the electrodes is increased. The importance of the degrading field concept, as represented by the foregoing disclosures, lies in the fact that the use of degrading electric fields is a significant step forward in circumventing the inherent limitations of prior art electric treating systems. This importance will become apparent from the discussion which follows.
At a given electric field strength, the magnitude of attractive forces between neighboring polarized or charged droplets of polar liquid in a non-polar, relatively nonconductive continuous liquid phase are dependent, in part, upon droplet radius. The smaller the droplet radii, the weaker will be the attractive forces which promote coalescence. Droplet radii further influence coalescence efficiency by affecting collision frequency. Droplets of polar liquid which carry a net charge will move in an electric field toward an oppositely charged electrode. At a given field strength and droplet charge, droplet velocity decreases with droplet radii. Reduced droplet velocity and small droplet radii combine to give reduced collision frequencies. Since the attractive force between oppositely charged droplets increases with decreasing distance between them, a decrease in collision, or encounter frequency, has a negative influence of coalescence efficiency. Refer to: Sadek, S. E. and Hendricks, C. D., I & EC Fund., 13, 139-142 (1974).
The attractive forces between polarized and charged droplets of polar liquid in a non-polar continuous phase, as well as the velocities of charged droplets, increase with increasing electric field strength to which the droplets are exposed. Therefore, it is not surprising that prior art has shown that very high field strengths are required to affect the coalescence of very small droplets of dispersed polar phase. However, at any given field strength there are certain limitations with respect to the maximum droplet size which may be achieved. Consider for the purpose of analysis, an emulsion of water in oil. The first of these limitations concerns the hydrodynamic forces to which water droplets are exposed in an electric field. The net charge acquired by water droplets results in motion of these droplets in the area between two charged electrodes, or between a charged electrode and ground. Superimposed on this motion are electroconvective currents generated in the continuous oil phase. These movements subject the water droplets to hydrodynamic forces which may cause individual droplets to disperse. The magnitude of hydrodynamic force required to disperse a water droplet is dependent, in part, upon droplet radius. The smaller the droplet radius, the greater the hydrodynamic force required to disperse the droplet.
The second of these limitations concerns electrical stresses on water droplets. Charged droplets have a tendency to disperse when a critical gradient is produced at the droplet surface. The magnitude of the critical dispersing gradient is inversely proportional to the square root of the droplet radius. Therefore, the larger the charged droplet the lower the electrical gradient at the droplet surface which will cause droplet dispersal. Excessive charge gradients at the droplet surface may be established through conductive or convective charging of the droplet, or through polarization of the droplet. Both hydrodynamic and electrical stresses then tend to establish a maximum water droplet size in the electric field. This maximum size will be determined by the physical properties of the two liquid phases and the strength and homogeneity of the electric field. The limitations of electrical coalescing systems have been discussed in scientific literature. Refer to: Sadek, S. E. and Hendricks, C. D., I&EC Fund., 13, 139-142 (1974); Waterman, L. C., Chem. Eng. Prog., 61 (10), 51-57 (1965); Doyle, A., Moffett, D. R., Vonnegut, B., J. Colloid Sci., 19, 136-143 (1964).
An emulsion of water or brine in crude oil will normally contain a distribution of aqueous phase droplet sizes. In oil field emulsions, for example, distributions in water droplet diameters of from 2-100 microns or more are not uncommon. In some emulsions the volume fraction of entrained water contained in the smallest droplets in the distribution can be significant enough to require their removal in order to produce a sufficiently dry oil. Coalescence of these very small water droplets requires high electric field strengths. However, as discussed previously, these high electric field strengths will limit the maximum droplet size achievable in the field. If water droplets of this limited size are of an insufficient size as to allow their gravitation from the oil, then little will be gained by their coalescence. In addition, any droplets of the original dispersion with dimensions greater than the maximum droplet size for the treating field strength will be dispersed. Prior to the appearance of the degraded field concept, most electric treaters used electric fields which allowed the production of water droplets which were sufficiently large to allow their gravitation from the oil, but were of insufficient strength to allow appreciable coalescence of the smallest droplets of the emulsion. In a degraded field created by widening the distance between charged electrodes in the direction of flow, the emulsion is first exposed to a high gradient zone which allows coalescence of the very small droplets. The product of this zone is then passed into a second zone which possesses a lower electrical gradient. The electric field in this second zone is of sufficient strength to promote coalescence of water droplets received from the first zone. However, the electric field in the second zone being weaker than that of the first zone allows an increase in the maximum droplet size which may exist in the field. Therefore, as the emulsion is passed progressively through electric fields of decreasing strength, the dispersed droplets grow in size until they are large enough to gravitate from the oil. The improvements over prior art offered by this type of degrading field are then, (1) more efficient coalescence of the very small droplets of typical water-in-oil emulsions, and (2) the creation of larger, more effectively gravitating water droplets in the treated mixture.
The embodiment of the degraded field concept described above offered significant improvement over prior art in electric treatment of organic continuous emulsions. However, it is not a panacea. Although the electric field strength decreases in the direction of flow, the charge on the electrodes remains relatively constant over its entire area. Therefore, mechanisms exist in low field strength areas for significant charging of water droplets by conductive or convective charge transfer and the subsequent dispersal of the droplet. Under the influence of electrical stresses, the droplet dispersal mechanism is such that the daughter droplets produced are much smaller than the original droplet. When these daughter droplets are produced in a low field strength area, there is little opportunity for their recoalescence. This problem is further compounded by the fact that critical dispersal gradients may be created in droplets near the electrodes in the exit region of the electric field through polarization of droplets. Strong polarization of the droplets can result from exposure to the locally strong inhomogeneous divergent electric fields at the electrode edges.
In addition to these basic limitations, the prior art also has the problem of both physically spacing plate electrodes to generate degrading electric fields, and a rather severe mechanical problem of suspension of these electrodes. The electrodes, in the form of metallic sheets, are heavy, making them difficult to support within a vessel. It is desirable that the composition of these electrodes be fabricated to reduce the weight and render it unnecessary for physical spatial divergence in the direction of flow of the liquid mixture through their passages.